Fundamentals of Organic Chemistry - Esters

1. Composition and structure of ester

An ester can be regarded as the product of the carboxylic acid molecule after the -OH is replaced by -OR '.

Abbreviated as RCOOR ', R and R´ can be the same or different. Functional groups are ester based pictures. The general formula of the ester formed by saturated monocarboxylic acid and saturated monocarboxylic acid is CnH2nO2(n≧2), so the carboxylic acid and ester have functional group isomerism.

Its representative is ethyl acetate, the molecular formula is C4H8O2, and the structural formula, structural simple formula and molecular structure are:

Taking ethyl acetate as an example, the method of writing the isomer of the ester is:

First write it as an ester isomerism, you can press formate, acetate, propionate... Write in the order of methyl ester, ethyl ester, propyl ester... Write the order, and take care to consider the carbon frame isomerism.

Then write the isomer of its carboxylic acid, also consider the carbon frame isomerism.

If you need to consider other isomers, such as hydroxyaldehydes, hydroxyketones, ether aldehydes, ether ketones, enols, and so on.

2. Physical properties, existence and naming of esters

Low esters are generally aromatic liquids, less dense than water, difficult to dissolve in water, easily soluble in organic solvents, many esters themselves are commonly used organic solvents.

Common esters are widely found in nature, such as amyl valerate in apple, ethyl butyrate in pineapple, isoamyl acetate in banana, etc. In daily life, esters are often added as spices in beverages, sweets and pastries.

The name of the ester is generally called "an acid an ester" according to the name of the corresponding acid and alcohol. For example, HCOOCH2CH3 is named ethyl formate,

The picture is called ethyl benzoate, the picture is called ethyl diacetate, and the picture is called diethyl oxalate.

3. Hydrolysis of ester (substitution reaction)

Under certain conditions, esters can be hydrolyzed to form carboxylic acids and alcohols. The hydrolysis of ester is the reverse reaction of esterification.

The C-O bond in the ester group is broken, and the C atom is connected to the OH atom to form an acid, and the O atom is connected to the hydrogen atom to form an alcohol.

Experiment:

(1) Experiments were designed to investigate the hydrolysis rate of ethyl acetate in neutral, acidic and alkaline solutions. (2) Experiments were designed to investigate the hydrolysis rate of ethyl acetate at different temperatures.

Experimental principle:

To explore the hydrolysis rate of ethyl acetate under different conditions, the principle of "single variable" can be used, that is, the control variable method. Ethyl acetate is insoluble in water, and its hydrolysis is reversible, so the hydrolysis rate can be judged by observing the difference in the time when the ester layer disappears.

Experimental operation:

(1) Take three test tubes and add 1mL ethyl acetate each, then add 5mL water, 5mL dilute sulfuric acid and 5mL dilute sodium hydroxide solution respectively. Heat in a water bath and observe the phenomenon.

(2) Take two test tubes, add 1mL ethyl acetate and 5mL dilute sodium hydroxide solution each, and heat them in a water bath at 40℃ and 80℃ respectively. Observe the phenomenon.

Experimental phenomena:

(1) After a period of time, the ester layer in test tube A is basically unchanged, the ester layer in test tube b is obviously thinner, and the ester layer in test tube c is disappeared. (2) The ester layer in the test tube at the water bath temperature of 80℃ disappeared first, and the ester layer in the test tube at 40℃ disappeared later.

Analysis and Conclusion:

(1) Ethyl acetate is basically not hydrolyzed under neutral conditions, and can be hydrolyzed under acidic conditions, but the hydrolysis rate is slower than that under alkaline conditions, and cannot be completely hydrolyzed, while under alkaline conditions, the hydrolysis is faster and can be completely hydrolyzed. (2) The higher the temperature, the greater the hydrolysis rate of ethyl acetate.

Consider: The hydrolysis of ethyl acetate under acidic conditions is reversible, why the hydrolysis under alkaline conditions is irreversible. Controlling experimental conditions (the principle of single variable) is an important method of scientific research. How do you experience this through the above experiments?

Under alkaline conditions, the base will neutralize the acetic acid generated by hydrolysis of ethyl acetate, reducing the concentration of the product and shifting the equilibrium of hydrolysis in the direction of the forward reaction, resulting in complete hydrolysis, that is, the reaction becomes irreversible.

In experiments, it is often necessary to design control experiments and follow the principle of single variable, that is, only one of the factors affecting the experiment should be different, so as to get a correct conclusion.

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